Nowadays, magnetic elements such as transformers and inductors are widely used in many electronic devices to generate induced magnetic fluxes. There are many methods for fabricating a magnetic element that is used in a switching power supply apparatus. According to a first method, a single-layered or three-layered insulated wire is wound around a magnetic core. According to a second method, after a coil pancake is produced by winding an insulated wire, the coil pancake is wound around a bobbin and then combined with a magnetic core. According to a third method, after an insulated wire is wound around a bobbin, the bobbin is combined with a magnetic core. Since the switching power supply apparatus has some switch elements, the magnetic element is readily suffered from an electromagnetic interference (EMI) problem. The EMI problem is detrimental to the neighboring circuits or electronic components. Especially, because of the parasitic capacitor between the primary side and the secondary side of the transformer, the transformer is a main reason of causing the EMI problem in the switching power supply apparatus.
For reducing the parasitic capacitor between the primary side and the secondary side of the transformer, a metallic shielding layer is usually arranged between the primary side and the secondary side and then the metallic shielding layer is connected to ground. FIG. 1A is a schematic view illustrating a transformer with a metallic shielding layer according to the prior art. FIG. 1B is a schematic cross-sectional view of the transformer shown in FIG. 1A and taken along the line AA. Hereinafter, a process for fabricating the transformer will be illustrated with reference to FIGS. 1A and 1B. First of all, a primary winding coil 14 is wound around a bobbin 11. Next, a metallic sheet 13 is sheathed around the primary winding coil 14. For preventing from generation of a short-circuited problem, a gap 131 is formed between both ends of the metallic sheet 13 (see FIG. 1B). Next, a secondary coil 12 is wound around the metallic sheet 13. Afterwards, a magnetic core is combined with the bobbin 11, thereby producing the transformer 1.
Although the transformer 1 is effective for reducing the EMI problem, there are still some drawbacks. For example, since the metallic sheet 13 fails to effectively isolate the primary winding coil 14 from the secondary coil 12, the EMI shielding efficacy of the transformer 1 is insufficient.
Therefore, there is a need of providing a coil assembly and a magnetic element with shielding functions so as to obviate the drawbacks encountered from the prior art.